Silicon (Si) photodiodes are commonly used light detectors comprising p- and n-type regions extending a distance into either a .pi.-type or .nu.-type Si body from opposed surfaces thereof. A p-n junction is formed at the interface between the remainder of the body and the region having opposite conductivity type to that of the body. In the operation of such a photodiode, a reverse bias voltage between the regions of opposite conductivity type creates a depletion region which extends through the body. Photons incident on the photodiode are absorbed creating electron-hole pairs which are swept out of the depleted region thereby generating a detectable electrical current.
In such devices portions of the surfaces are coated with a thermally grown SiO.sub.2 passivation layer to reduce the sensitivity of the surface to its ambient. It is well known, however, that the presence of this SiO.sub.2 layer induces the formation of an n-type surface channel in an underlying high resistivity .pi.-type Si surface. Mills, in U.S. Pat. No. 4,009,058, has disclosed a n-.pi.-p.sup.+ photodiode in which a p-type surface control layer extends into the body from that portion of the surface contiguous with that portion from which the n-type region extends. The presence of this layer prevents the induction of an n-type surface channel by over-compensation and provides a p-n junction with the n-type region at the surface. In a p-.nu.-n photodiode the presence of an n-type surface channel would enhance the n-type nature of that portion of the surface outside of the p-type region and thus a control layer should not be required.
In Si photodiodes used in a photovoltaic mode a parameter of interest and importance is the junction resistance which is defined as the slope of the V-I curve as it passes through the point V=I=0. The junction resistance can be defined as EQU R=(dV/dI).sub.V=0 =nkT/qI.sub.o
where n is a number between 1 and 2, k is Boltzmann's constant, T is the temperature, q is the electronic charge, and I.sub.o is the junction current at an applied reverse bias voltage greater than a few times kT. For a p-.nu.-n Si photodiode where the resistivity of the Si body is in excess of about 1000 ohm-cm (.OMEGA.-cm), the junction current I.sub.o is large and the corresponding junction resistance is low. We have discovered that a large fraction of the high junction current and corresponding low junction resistance is due to the generation of holes in the n-type surface channel at or near the interface with the passivation layer and the flow of these holes across the p-n junction.